Rate 8/9 constrained codes for partial-response class-IV signaling with maximum-likelihood sequence detection (PRML) have been designed to achieve proper operation of gain and timing control loops and reduction of path-memory length of Viterbi detectors in PRML receivers. In the presence of a precoder between partial-response class-IV (PR-IV) channel and rate 8/9 encoder, these two objectives are met by restricting the maximum run-length of 0's, denoted by G.sub.0, in encoded binary sequences {b.sub.n } and limiting the maximum number of consecutive 0's, denoted by I.sub.0, in the two subsequences of {b.sub.n } with even or odd time indices. An example of a system where G.sub.0 and I.sub.0 are both limited by appropriate coding (either to 4 and 4, or to 3 and 6, respectively) is disclosed in U.S. Pat. No. 4,707,681 to Eggenberger.
It would be desirable to have coding schemes which can accomplish a third task, i.e. to speed up the start-up procedure of PRML receivers by shortening the minimum observation length required to distinguish reliably encoded user data from the timing preamble {1,1,1, . . . , 1,1}, which in many systems is part of training sequences preceding encoded data sequences {b.sub.n }.